Erwinia Chrysanthemi (Dickeya Spp.) What It Is, and What You Can Do

Erwinia chrysanthemi (Dickeya spp.) What it is, and what you can do

The potato erwinias can all cause tuber soft rots, but only Erwinia carotovora subspecies atroseptica (Eca) and Erwinia chrysanthemi (Ech) produce economically damaging disease in the growing crop. Eca is the main cause of blackleg, but Ech causes sporadic stem rots in warmer parts of Great Britain when the temperature rises above 25ºC. It appears to be spreading on potatoes in Europe and reached England by 1990 . In common with other potato erwinias Ech is mostly spread via seed but can be controlled through good hygiene. The Safe Haven Certification Scheme, originally designed to protect against ring rot introduction, offers significant benefits for controlling the spread of all bacterial diseases, including Ech.

Ech (or Dickeya dianthicola, the species that has been found in GB) has the potential to cause high levels of wilting in potato crops. This is particularly evident in the warmer southerly potato growing regions of Europe, but incidences in Britain are becoming more common as we experience warmer spring and summer conditions. It has not been found in Scotland to date. There is also evidence that low tuber inoculum levels can cause significant disease. The factors influencing disease development are generally the same as for blackleg.

Best practice 1. BPC has published a disease risk assessment guide for blackleg and soft rot which is directly applicable to Ech (Dickeya spp.). In summary: • Check varietal susceptibility • Avoid de-sprouting at planting • Remove rots and clean grader prior to • Do not over-irrigate grading • Harvest crop as early as possible • Avoid poorly-drained fields • Minimise damage at harvest • Avoid short rotations • During early storage ventilate • Use diagnostics to test seed stocks for using dry air Erwinia (but see cautionary note in guide)

See Managing the risk of blackleg and soft rot published by BPC*, January 2007.

British Potato Council, 4300 Nash Court, John Smith Drive, Oxford Business Park South, Oxford, OX4 2RT Erwinia chrysanthemi (Dickeya spp.) – What it is, and what you can do

2. There is a major opportunity to reduce the risk of introducing Dickeya spp. into Britain through sourcing seed via the “Safe Haven Certification Scheme”, which was originally introduced to protect British growers from ring rot (Clavibacter michiganensis subsp. sepedonicus). This scheme involves minimal bureaucracy and reduces risk through a series of best practice standards covering all of the infection points. Combined with existing seed classification schemes and phytosanitary controls, the Safe Haven Certification Scheme provides the best chance of achieving exclusion.

Action • Obtain and read the BPC’s risk guide Managing the risk of blackleg and soft rot. Available from BPC Publications.*

• Read about the Safe Haven Certification Scheme – there is an 8 page guide available on the BPC website, or from BPC Publications.*

Seed growers: Consider joining the scheme. Ware growers: Consider buying exclusively Safe Haven seed. See also: www.assuredproduce.co.uk/safehaven

• Obtain and read the BPC’s report: Erwinia chrysanthemi (Dickeya spp.) – The Facts written by Dr John Elphinstone of the Central Science Laboratory, and Dr Ian Toth of the Scottish Crop Research Institute. Available on the BPC website, or from BPC publications.*

* BPC Publications tel: 01865 782222 (24 hours), fax: 01865 782283, email: [email protected], website: www.potato.org.uk

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Executive summary from: “Erwinia chrysanthemi (Dickeya spp.) – The Facts”

The pathogen Erwinia chrysanthemi (Echr) is a complex of different bacteria now reclassified as species of Dickeya. While D. dadantii and D. zeae (formerly Echr biovar 3 or 8) are pathogens of potato in warmer countries, D. dianthicola (formerly Echr biovar 1 and 7) appears to be spreading on potatoes in Europe. The revised nomenclature of these pathogens has distinguished them from other soft rot erwiniae (including P. atrosepticum and P. carotovorum). Symptoms Symptoms of soft rot disease on potato tubers are similar whether caused by Dickeya or Pectobacterium spp. In the field, disease develops following movement of either pathogen from the stem base. Whereas P. atrosepticum typically causes blackleg symptoms under cool wet conditions, symptoms due to Dickeya spp. have been more commonly observed to occur under warm conditions (when temperatures exceed 25 ºC). The foliar symptoms most commonly associated with D. dianthicola in warm dry growing conditions include brown staining of the vascular tissues and occasionally necrosis and hollowing of the stem, which usually remains green until leaf desiccation is complete. Symptoms due to Dickeya spp. are also thought to occur later in the season. However, there is some dispute as to whether disease symptoms and timing alone can differentiate the two pathogens. Symptoms caused by D. dianthicola under warm dry conditions can be confused with those of other wilting diseases. Geographic distribution Dickeya spp. were first reported on potato in Europe in the Netherlands in the 1970s and has since been reported on potato in a number of other European countries. To date all European potato isolates appear to be D. dianthicola, although other Dickeya spp. have been found on potato in other countries including Australia and Peru. The pathogen has been reported worldwide on many hosts as Erwinia chrysanthemi, but the corresponding Dickeya spp. has yet to be determined in most cases. Over 40 outbreaks of D. dianthicola in England have been officially confirmed by laboratory testing at CSL since 1990, and is probably more widespread in potato crops in England and Wales than official records suggest. Most cases appear to be from imported seed but at least one was from UK-produced seed. To date there have been no findings of Dickeya spp. on potatoes in Scotland. Biology, survival and dissemination of the pathogen Factors influencing disease development on potato caused by Dickeya spp. are generally the same as for P. atrosepticum, with the exception of temperature, where a warmer spring and summer favours disease development by Dickeya spp. A lower level of inoculum, irrigation from contaminated water courses and more rapid movement through the vascular system of the plant may also favour disease caused by Dickeya spp. over P. atrosepticum. The most important means of dissemination for potato is movement of latently infected seed tubers. In other host plants, of which there is a wide range, spread over long distances and especially across borders, is mainly via infected vegetative material. Dickeya spp. have been identified in water courses in several countries and in one case in Sweden on the riparian weed Solanum dulcamara. Occurrence in GB watercourses or S. dulcamara is unknown. The high host diversity across the Dickeya species suggests that wild host plants could play an important role in survival. In plant-free soil, survival is less than 6 months and, therefore, over-wintering is unlikely.

Assessment of risk and economic loss Unpublished UK studies found that D. dianthicola was highly contagious and aggressive with low tuber inoculum levels leading to high wilting incidence in field plots. However, losses due to potential infections by Dickeya spp. other than D. dianthicola are not expected to be significant under UK growing conditions, although seed infections may result in significant disease levels if seed is exported to warm climates or our own climate warms. Control and diagnostics Dickeya, like P. atrosepticum, is regarded as a seed-borne pathogen and is controlled largely through seed classification in line with domestic and EU legislation. In the UK, as in other European countries, the seed potato classification schemes set tolerances for diseases encountered during visual inspections of growing crops and harvested tubers. There is no official post-harvest testing programme, although voluntary testing services provide useful decision support. On-farm control measures for Dickeya spp. are currently the same as for P. atrosepticum, largely because there is insufficient data available to support alternative Dickeya-specific measures. However, where information is available, it suggests that the use of diagnostics, avoiding over-irrigation and controlling secondary hosts may be a way to avoid disease caused by Dickeya spp. In other European countries, as in the UK, there are no Dickeya spp.-specific control measures and no compulsory testing in operation. Some countries attempt to differentiate Dickeya spp. and P. atrosepticum based on visual inspection, while others also use diagnostics but on a voluntary basis. There are media-, antibody- and PCR-based diagnostics available for Dickeya spp. and in some cases for the soft rot erwiniae as a group. All three method types are used throughout Europe although PCR-based methods appear to be the most reliable. Most countries do not differentiate between these pathogens but consider disease as caused by “soft rot erwiniae” and use control measures accordingly. A major opportunity we have to reduce the risk of introducing Dickeya spp. into the UK is by growers joining the new “Safe Haven” Scheme. Threats D. dianthicola now appears to be as important on potato as P. atrosepticum in several Northern European countries, and experts consider it to be increasing in importance. It is very likely to pose a threat to UK potato production and has already been detected in an English seed crop. The range of wilting symptoms due to D. dianthicola estimated in English crops has varied from <1% to 20-30%. Currently, the protected Scottish seed potato regions appear to be clear of this pathogen but for how long remains to be seen. Effective control measures implemented now are our best, and possible only, chance of preventing economic losses caused by this pathogen as it gains a foothold in the UK. Opportunities Demonstrating that seed-growing areas in the UK are free from Dickeya would provide a competitive advantage for GB seed exports. There may also be added advantages to GB ware potato growers, particularly in warm growing seasons, in having access to Dickeya-free seed supplies (e.g. through safe-havens accredited seed stocks).